Green emitting phosphor

ABSTRACT

An efficient green emitting phosphor results from the coactivation of yttrium fluoride with gadolinium and erbium. The excitation energy absorbed by the Gd 3+  centers is transferred non-radiatively to minority Er 3+  centers, whose emission produces green light when excited by 275 nanometer or 378 nanometer radiation.

FIELD OF THE INVENTION

This invention relates to a green emitting phosphor. More particularly,this invention relates to an efficient green emitting phosphor resultingfrom the coactivation of yttrium fluoride with gadolinium and erbium.

BACKGROUND OF THE INVENTION

Recently literature reports, such as J. Th. W. de Hair, "Theintermediate role of Gd³⁺ in the energy transfer from a sensitizer to anactivator", Journal of Luminescence 18/19, (1979) 797-800, J. Th. W. deHair and W. L. Konijnendijk, "The intermediate role of Gd³⁺ in theenergy transfer from a sensitizer to an activator (especially Tb³⁺)",Journal of Electrochem Society, Vol. 127, #1, (1980) 161-164, T. E.Peters, R. G. Pappalardo, and R. B. Hunt Jr., "Unusual green emissionfrom Mn²⁺ and Gd(BO₂)₃ ", Journal of Luminescence 31 and 32 (1984290-292), have described how electronic excitation-energy is efficientlytransferred in oxide hosts from Gd³⁺ centers to coactivators capable ofvisible emission, such as Tb³⁺, Dy³⁺ and Mn²⁺. However, there is nomention of efficiently transferring non-radiatively excitation energyabsorbed by Gd³⁺ centers to minority Er³⁺ centers, whose emissionproduces green light as disclosed in the present invention.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a new andimproved green-emitting phosphor exhibits a characteristic peak emissionfrom about 510 to about 560 nanometers when excited by about 275nanometer excitation. The phosphor has a composition defined by thegeneral formula:

    Y.sub.1-0.65-x Gd.sub.0.65 Er.sub.X F.sub.3

where p1 0.005≦x≦0.05.

In accordance with one aspect of the present invention, a new andimproved green-emitting phosphor exhibits a characteristic peak emissionfrom about 510 to about 560 nanometers when excited by about 378nanometer excitation. The phosphor having a composition defined by thegeneral formula:

    Y.sub.1-0.65-x Gd.sub.0.65 Er.sub.X F.sub.3

where 0.005≦x≦0.05.

In accordance with still another aspect of the present invention, a newand improved method of making a green-emitting phosphor comprises thefollowing steps:

Step 1--Gadolinium fluoride, erbium fluoride, yttrium fluoride, andammonium fluoride are blended to form a mixture.

Step 2--The mixture of step 1 is heated in a vitreous-carbon crucible ina stream of dry nitrogen gas to a temperature and for a time sufficientto melt the mixture to form a melt.

Step 3--The melt from step 2 is cooled to form a fused product.

Step 4--The fused product from step 3 is ground to form a ground powder.

Step 5--The ground powder from step 4 is sieved to form a screenedpowder.

Step 6--The screened powder from step 5 is excited with an ultravioletlight source to obtain a green emitting phosphor.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings

FIG. 1 depicts the emission spectrum for a phosphor composition inaccordance with one aspect of the present invention when excited by a275 nanometer source.

FIG. 2 depicts an excitation spectrum for a phosphor composition inaccordance with one aspect of the present invention.

FIG. 3 depicts an emission spectrum for a phosphor composition inaccordance with one aspect of the present invention when excited by a378 nanometer source.

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above described drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A phosphor of the present invention, in one specific example, wasprepared by blending 11.14 grams of gadolinium fluoride, 0.18 grams oferbium fluoride, 3.97 grams of yttrium fluoride, and 0.76 grams ofammonium fluoride (approx. 5 wt.% NH₄ HF₂). The resulting mixture wasthen heated in a vitreous-carbon crucible to a temperature in excess ofmelting point of the mixture, approx. 1330° C. for approx. 30 minutes ina stream of dry nitrogen gas. After the material was melted and cooledto room temperature, the fused product was easily removed from thevitreous-carbon crucible, ground to a powder and sieved through a 200mesh nylon screen. The above process yielded a phosphor powder ofcomposition Y₀.34 Gd₀.65 Er₀.01 F₃ having an orthorhombic crystalstructure. The resulting phosphor powder exhibited an emission spectrumas depicted in FIG. 1 when excited by a 275 nanometer wavelength andexhibited an emission as depicted in FIG. 3 when excited by a 378nanometer excitation wavelength.

The 275 nanometer excitation of the phosphor of the present inventionwhere there is a relatively strong absorption by the Gd³⁺ centers and avery weak absorption by the Er³⁺ centers, produces the emission spectrumdepicted in FIG. 1. The emission consists of groups of emission linesoccurring in several regions of the UV and visible spectrum. Since Gd³⁺is known to exhibit only emission in the ultraviolet spectral region, itis evident that the visible emission must be produced by the Er³⁺centers.

The narrow emission lines in the ultra violet, at approx. 315 nanometersdepicted in FIG. 1, are the typical emission from Gd³⁺ centers. Incontrast, the sharp emission lines in the blue, at approx. 400nanometers and approx. 460 nanometers; the intense group spanning thegreen spectral region from approx. 510 to approx. 560 nanometers; andadditional lines in the red and far red region (not shown in FIG. 2) allrepresent emissions from the Er³⁺ centers. This is confirmed byexcitation spectra for green emission at 544 nanometers, shown in FIG.2, and revealing that in the short ultra violet spectral regionexcitation of the emission is mainly associated with absorption by the(majority) Gd³⁺ centers.

If the material is excited at approx. 378 nanometers, one of theexcitation regions depicted in FIG. 2, the resulting emission spectrum(FIG. 3) does not contain the emission group at approx. 470 nanometers.This difference is explained on the basis of the energy level scheme forEr³⁺ , keeping in mind that the presence of Er³⁺ at low concentrationsas is the case here, and in a fluoride host, reduces the probability forinter-level non-radiative relaxation within the Er³⁺ sublattice.

An additional interesting effect is the change in relative emissionintensity from FIG. 1 to FIG. 3, both for the emission group at approx.400 nanometers, and for the emission group in the green spectral region.It is contended that the narrow emission lines, generally missing fromthe emission spectrum of FIG. 3, originate from the ² P_(3/2) level ofEr³⁺, which is expected to be close in energy to the Gd³⁺ emitting levelat approx. 310 nanometers, and which is most likely the acceptor levelin the Gd³⁺ to Er³⁺ energy transfer.

The visible emission from ² P_(3/2) of Er³⁺ to intermediate excitedlevels of Er³⁺, located in energy in the visible spectral region, can inturn lead to a Photon Cascade Emission process, whereby a single ultraviolet photon absorbed by Gd³⁺ is transferred to the ² P_(3/2) level ofEr³⁺, and converted by the latter center into two visible photons.

In addition to the above formulation utilizing the preferred 0.01 molesof erbium, the erbium formulation was varied from about 0.005 moles oferbium to about 0.05 moles of erbium in which the yttrium concentrationwas varied in accordance with the following formula: Y₁₋₀.65-x Gd₀.65Er_(x) in which x represents the concentration of the erbium in moles.

Table I shows the relative emission intensity as a function of theerbium concentration in which the emission of the material containing0.01 moles of erbium was normalized at 100.

                  TABLE I                                                         ______________________________________                                        Normalized Emission Intensity                                                 from Y.sub.1-0.65-x Gd.sub.0.65 Er.sub.x                                      (for 275 nm excitation)                                                       ______________________________________                                        Y.sub.0.345 Gd.sub.0.65 Er.sub.0.005                                                           60                                                           Y.sub.0.34 Gd.sub.0.65 Er.sub.0.01                                                             100                                                          Y.sub.0.32 Gd.sub.0.65 Er.sub.0.03                                                             76                                                           Y.sub.0.030 Gd.sub.0.65 Er.sub.0.05                                                            40                                                           ______________________________________                                    

As depicted in FIG. 1 the new green emitting phosphor of the presentinvention has an intense green spectrum distribution from approx. from510 to approx. 560 nanometers when excited with 275 nanometerwavelength.

While there has been shown and described what is at present consideredthe preferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modification may be madetherein departing from the scope of the invention as defined by theappended claims.

What is claimed is:
 1. A green emitting Gd³⁺ and Er³⁺ coactivatedphosphor exhibiting a characteristic peak emmission from about 510 toabout 560 nanometers when excited by about 275 nanometers or about 378nanometers excitation, said phosphor having a composition defined by thegeneral formula:

    Y.sub.1-0.65-x Gd.sub.0.65 Er.sub.x F.sub.3

where 0.005≦x≦0.05.
 2. A green emitting phosphor in accordance withclaim 1 wherein said x is about 0.01.
 3. A method of making a greenemitting Gd³⁺ and Er³⁺ coactivated phosphor exhibiting a characteristicpeak emission from about 510 to about 560 nanometers when excited byabout 275 nanometers or about 378 nanometers excitation, said phosphorhaving a composition defined by the general formula: Y₁₋₀.65-x Gd₀.65Er_(x) F₃ where 0.005≦X≦0.05 comprising the following steps:Step1--blending gadolinium fluoride, erbium fluoride, yttrium fluoride, inamounts corresponding to said general formula and ammonium fluoride toform a mixture; Step 2--heating said mixture of step 1 in avitreouscarbon crucible in a stream of dry nitrogen gas to a temperatureand for a time sufficient to melt said mixture to form a melt; Step3--cooling said melt from step 2 to form a fused product; Step4--grinding said fused product to form a ground powder; and Step5--sieving said ground powder to obtain said phosphor
 4. A method inaccordance with claim 3 wherein said temperature is about 1330° C.
 5. Amethod in accordance with claim 3 wherein said time is approximately 30minutes.